A complementary strategy of utilizing ππ* excited state of alkene instead of nπ* excited state of the carbonyl chromophore in a “transposed Paternò–Büchi” reaction is evaluated with atropisomeric enamides as the model system. Based on photophysical investigations, the nature of excited states and the reactive pathway was deciphered leading to atropselective reaction. This new concept of switching of excited-state configuration should pave the way to control the stereochemical course of photoreaction due to the orbital approaches required for photochemical reactivity.

•Thioxanthone-benzothiophenes absorb in the visible spectral region.•These new conjugates show high extinction coefficients and high triplet yields.•The triplet states react with tertiary amines to generate free radicals.•These compounds are efficient photoinitiators for free radical polymerization.Photoinitiators for free radical polymerization based on the thioxanthone chromophore that contains benzothiophene were synthesized and characterized. Compared to thioxanthone, these compounds show a bathochromic shifted absorption up to ∼460 nm. High quantum yields for intersystem crossing generate sufficient amounts of triplet states. Initiator radicals are generated by reaction of the triplet states with tertiary amines, such as diethanolamine with high rate constants (2–6 × 109 M−1 s−1) as determined by laser flash photolysis. Photoinitiated polymerization experiments of MMA showed efficient polymerization with initiator concentrations as low as 0.1 mM.

Endogenous chromophores in human skin, when exposed to sunlight, generate harmful reactive oxygen species (ROS). Protoporphyrin IX (PpIX) is one of the common chromophores in human tissue. A series of aromatic cyanoacrylates were tested as quenchers of excited singlet and triplet states of PpIX. While the diaryl cyanoacrylate (1) did not quench excited singlet or triplet states of PpIX, some cyanoacrylates with fused aromatic rings showed excited singlet state quenching rate constants as high as 5 × 109 M−1 s−1 (acetonitrile solution). In addition, one of the fused ring cyanoacrylates (5) quenches PpIX triplet states with a rate constant of 3 × 109 M−1 s−1. The observed quenching rate constants correlated well with the suppression of singlet oxygen generation from PpIX under visible light exposure in the presence of dissolved oxygen. This photostabilization of endogenous chromophores can prevent or reduce ROS generation and perhaps constitute a new approach to mitigating cutaneous oxidative stress.

Time-resolved electron paramagnetic resonance spectroscopy, transient absorption, and phosphorescence spectroscopy were used to investigate the spin polarization of a nitroxide free radical induced by interaction with singlet oxygen (1O2). The latter was generated by photolysis of endoperoxides of two anthracene derivatives. Although both anthracene endoperoxides are structurally similar, opposite spin polarization of the nitroxide was observed. Photolysis of one endoperoxide leads to absorptive nitroxide spin polarization due to interaction with the generated 1O2. Photolysis of the other endoperoxide generated emissive nitroxide spin polarization, probably due to interaction of the endoperoxide triplet states with nitroxides.

Nonbiaryl atropisomeric acrylimides underwent facile [2 + 2] photocycloaddition leading to cross-cyclobutane adducts with very high stereospecificity (enantiomeric excess (ee): 99% and diastereomeric excess (de): 99%). The photoreactions proceeded smoothly in isotropic media for both direct and triplet sensitized irradiations. The reactions were also found to be very efficient in the solid state where the same cross-cyclobutane adduct was observed. Photophysical studies enabled us to understand the excited-state photochemistry of acrylimides. The triplet energy was found to be ∼63 kcal/mol. The reactions proceeded predominantly via a singlet excited state upon direct irradiation with very poor intersystem crossing that was ascertained by quantification of the generated singlet oxygen. The reactions progressed smoothly with triplet sensitization with UV or visible-light irradiations. Laser flash photolysis experiments established the triplet transient of atropisomeric acrylimides with a triplet lifetime at room temperature of ∼40 ns.

A photoinitiator for free-radical polymerization based on a thioxanthone chromophore containing two acetic acid functions was synthesized and characterized. Photophysical studies such as fluorescence, phosphorescence, and laser flash photolysis in addition to photopolymerization of acrylates were performed to elucidate the radical generation mechanism involving intramolecular electron transfer from the triplet state followed by decarboxylation. We found that the position of the acetic acid substituent is critical for the photoreactivity. In most solvents and acrylic monomers, if the acetic acid functionality is at the 1-position, the singlet excited states are deactivated rapidly before electron transfer can occur, resulting in negligible photoreactivity. The excited-state deactivation probably involves intramolecular H-bonding deactivation. The intramolecular H-bonding is disrupted by solvents that support intermolecular H-bonding, such as DMF and DMSO, leading to efficient intramolecular photoreaction.

Photoinduced electron transfer reactions of highly conjugated thiophene derivatives, 4,7-di(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)benzo[1,2,5]thiadiazole (DTDT) and 5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2,3-di(thiophen-2-yl)quinoxaline (DTDQ), with diphenyliodonium hexafluorophosphate (Ph2I+PF6–) and triphenylsulfonium hexafluorophosphate (Ph3S+PF6–) were investigated by laser flash photolysis, fluorescence and phosphorescence spectroscopy, and polymerization studies. High fluorescence quantum yields, long fluorescence lifetimes (∼10 ns in aprotic solvents), and absence of detectable phosphorescence at 77 K for both compounds indicate inefficient intersystem crossing into the triplet state and dominant role of singlet excited state. Photolysis of DTDT or DTDQ in the presence of iodonium salt with visible light results in the formation of radical cations of DTDT and DTDQ as detected by laser flash photolysis. Sulfonium salts do not undergo such electron transfer reactions due to the unfavorable redox potentials. The importance of the described photoinduced electron transfer process with respect to the initiation of cationic polymerization and formation of conjugated polymers was demonstrated.

The initiation efficiencies of photoinitiators for free radical polymerization, 2,4,6-trimethylbenzoyldiphenylphosphine oxide (TMDPO) and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (BAPO), are enhanced by addition of mercaptothioxanthone (TX-SH) to the initiator formulation. The additive TX-SH combines the benefits of a triplet sensitizer (enhanced light absorption) and thiol functionality (reduced oxygen inhibition). On the basis of polymerization studies, laser flash photolysis, fluorescence, and phosporescence, the following mechanism is proposed: most of the light is absorbed by TX-SH to generate 3(TX-SH)*, which transfers its energy to TMDPO and BAPO followed by α-cleavage to produce the initiating radicals. In addition, excellent polymerization yields were observed in air-saturated solutions when TX-SH was added. With the addition of TX-SH to initiation formulations, the inhibition due to oxygen is reduced, probably by the generation of reactive thiyl radicals by hydrogen abstraction of inactive peroxyl radicals from the thiol functionality.